Wednesday, 3 July 2019

Some design tweaks and finish machining for the Bridgeport Z axis assembly

What's wrong, Fatty?
Most of the key compts are now pretty much complete. Overall I'm reasonably happy with the design concept and the relative simplicity(?) and ease of manufacture. However, some additional finessing / fettling will be required before I'm happy - and indeed before the parts will go together.

Thrust bearing:
The original design had a deep groove bearing (radial and axial thrust) at each end of the ballscrew. However, that isn't an approved means of supporting a ballscrew, since one end needs to be floating unless you want an indeterminate preload, possibly including zero preload (= backlash territory). Furthermore, there's really no benefit in having a bearing at the top of the ballscrew. I ended up shimming the bottom bearing on the first assembly but that's crap and fiddly.

For the revised scheme, I went with one bearing - at the bottom of the ballscrew. Obviously this needs to be a dual row bearing, so it can take bidirectional axial loads as well as the radial loads from the belt drive. Stupidly, I left the specification(?) of the bearing to The Stupid Fat Bloke. He simply carried over the existing Chinesium "5201" deep groove bearing. 

The 5201 bearing is a 12mm ID / 32mm OD bearing with an O/A length of 16mm. That's way OTT for what I want / need.

The 4201 is very similar (same ID and OD) but is shorter at 14mm. 
I like the look of that better and have ordered some SKF branded examples from Bearingboys

That of course requires some modification to the design of the motor bracket.....

Remachining the motor bracket bearing housing:
So now I have to shorten the length of the bearing bore by ~2mm. However, I machined the bore to a nominal 32mm and although it came out very nicely, The Stupid Fat Bloke didn't bother to actually check if the bearing would fit in the hole. Details, eh. 

The bearing can just about be forced in with the application of a lot of force / blows but that's not the way to do things. I need to machine a few 10s of microns from the bore. I suspect some of the problem is due to slight noncircularity(?).

In fact it's quite easy to create a new setup in Fusion to allow you to go back in and finesse the final dimensions / modify the design - as long as this only involves material removal of course.

In order to be able to drift the bearing out, it makes sense to reduce the diameter of the shoulder, so I'll do this at the same time.

In the new setup, select the final CAD model as the stock, then define a convenient local origin. In my case, I simply picked up the axis of the bore and the internal flat surface of the housing using the Renishaw probe. The operation doesn't look as if it should be removing any material but in fact it works fine. 

Bearing retainer bracket:
Having shortened the length of the bearing by 2mm, I can increase the thickness of the retainer bracket accordingly. It could do with beefing up somewhat and this is an ideal opportunity to do so. That also requires me to be back in and increase the depth of the cavity in the housing that receives the bracket. The process is similar - modify the CAD model, then apparently cut air. In this operation, I went for a 3D Adaptive to rough out the bulk oif the material, then a 2D Contour to provide a nice finish:

Picking up the part origin using the Renishaw is deadly accurate in comparison with the tolerances I actually get from the machining operation itself.

Went nicely. Sorted.

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